AM/FM radio troubleshooting
klem kedidelhopper
transistors. I've had this thing for over forty years and although I
don't use it often it has been a nice radio. In 1971 I replaced the
transistor in the FM detector circuit and I have had no problems with
it until just recently. Last month I spent three days in the hospital
and I brought the Grundig with me. I noticed that after it was on for
about a half hour it would start to cut out. I turned it off and then
later turned it on again, and after a period of time it happened
again. After several similar episodes I stopped using it. I wondered
about some type of interference from the hospital but that was wishful
thinking. In any case being in no position to test it I decided to
just put it away. Yesterday I put it on the bench and took a look at
it. Sure enough after a brief time it cut out again. I injected a
signal, (my finger on a small screwdriver) onto the wiper of the
volume control and ascertained that there is nothing wrong up to the
speaker. So now I need to get into the IF's and detector. My problem
is my old signal generators are not working and all I can come up with
for 10.7MHZ. is a CW signal out of my MFJ249 SWR meter. The MFJ puts
out a clean signal however it is CW, and it's amplitude is not
adjustable. I don't think that an unmodulated signal would be a
problem in this application but I don't know what the amplitude of the
MFJ is and I would hate to blow up an otherwise good IF amplifier with
an excessive signal. I''m admittedly a bit rusty having not trouble
shot an IF amplifier/detector circuit in many years so does anyone
have a feel for how I might signal trace this radio with what I have
on hand? Thanks very much for any advice. Lenny
Phil Allison
"klem kedidelhopper has six toes "
** Might be another dodgy chemical fuse - eh ??
.... Phil
spamtrap1888
wrote:
> I have a Grundig "Transistor 305" portable radio. It uses germanium
> transistors. I've had this thing for over forty years and although I
> don't use it often it has been a nice radio. In 1971 I replaced the
> transistor in the FM detector circuit and I have had no problems with
> it until just recently. Last month I spent three days in the hospital
> and I brought the Grundig with me. I noticed that after it was on for
> about a half hour it would start to cut out. I turned it off and then
> later turned it on again, and after a period of time it happened
> again. After several similar episodes I stopped using it. I wondered
> about some type of interference from the hospital but that was wishful
> thinking. In any case being in no position to test it I decided to
> just put it away. Yesterday I put it on the bench and took a look at
> it. Sure enough after a brief time it cut out again. I injected a
> signal, (my finger on a small screwdriver) onto the wiper of the
> volume control and ascertained that there is nothing wrong up to the
> speaker. So now I need to get into the IF's and detector. My problem
> is my old signal generators are not working and all I can come up with
> for 10.7MHZ. is a CW signal out of my MFJ249 SWR meter. The MFJ puts
> out a clean signal however it is CW, and it's amplitude is not
> adjustable. I don't think that an unmodulated signal would be a
> problem in this application but I don't know what the amplitude of the
> MFJ is
Somehow I feel looking at an oscilloscope might help with determining
the amplitude of the signal.
and I would hate to blow up an otherwise good IF amplifier with
> an excessive signal.
Somehow I feel that an excessive signal could be tamed with a resistor
divider circuit. Knowing what the input impedance of IF circuits is
might help.
> I''m admittedly a bit rusty having not trouble
> shot an IF amplifier/detector circuit in many years so does anyone
> have a feel for how I might signal trace this radio with what I have
> on hand? Thanks very much for any advice. Lenny
One source of IF signal might be another FM radio. Just a wild, crazy
thought.
Further, if this is an AM-FM as the subject header states, you forgot
to mention if the AM worked.
Robert Macy
I would first look at mechanical failure of the connections. ...PCB/
traces/etc.
Especially around the FM Detector area where you heat shocked the PCB
when changing that transistor.
I know it's hard to believe battery can heat that much but with age on
a PCB, you'd be surprised. Ever try cold spray when radio failed?
N_Cook
news:d384fa64-4fcf-42c3...@p23g2000vbl.googlegroups.com...
Any 4 pin Ge transistors in there? if there is then the usual simple fudge
is worth a try
klem kedidelhopper
> klem kedidelhopper <captainvideo462...@gmail.com> wrote in message
Well the AM is not working either when this set fails. And since the
set uses the same transistors but different coils for each stage that
doesn't really help me to track this down. I could look at the CW
signal from the 249 with my 50MHZ. scope or my spectrum analyzer but I
still don't know what a safe signal amplitude would be. Does anyone
know what the proper typical amplitude of an IF signal would be so
that I would be able to simulate this and inject this signal into the
IF amplifiers and detector stage? BTW except for the outputs, they're
all 4 pin GE transistors. What do you mean by the "simple fudge"?
Thanks, Lenny
William Sommerwerck
value larger than the net forward bias on the IF transistors. You can be
reasonably certain a signal of that level will not damage the transistors.
John-Del
> ** Might be another dodgy chemical fuse - eh ??
>
> .... Phil
Fuses, chemical or not, don't become intermittent.
Idiot.
John
Ian Field
Does it have the old TO7 transistors like OC170/171/AF116/117 etc?
They're prone to internal whiskering, most commonly the can/screen to one of
the 3 electrodes, lifting the screen lead might get it going, some people
zap the whiskers with 47uF charged to 50V (twist C,B & E together first).
If the whiskers have reached 2 or more electrodes you're pretty much
stuffed, some people use BF450/451 as replacements but you usually have to
tweak the base bias resistors to suit silicon 0.7V and you might have to
align out the different collector capacitance (do one stage at a time).
If you have extreme manual dexterity and bloody good eyesight you can melt
the solder seal between can & header and put the TO92 inside the TO7 can to
preserve the appearance of originality.
Phil Allison
"John-Del"
"Phil Allison"
> ** Might be another dodgy chemical fuse - eh ??
>
Fuses, chemical or not, don't become intermittent.
** Then you have not seen everything.
Imbecile.
... Phil
Michael A. Terrell
Check the collector voltages on the IF and mixer stage. Use at least
a 1 meg resistor between the probe and the transistor to keep from
disturbing the circuit. I saw a lot of this in old germanium transistor
car radios. Noise induced into the circuit would momentarily restore
the transistor enough to work for a while, but of there was zero
collector voltage, one of the IF transformers was open. If it was high,
the transistor was bad. I troubleshot those radios and repaired most of
them in under five minutes with that simple method. :)
--
It's easy to think outside the box, when you have a cutting torch.
klem kedidelhopper
wrote:
The IF's are AF124,and 126. And to answer a previous question I did
use cold spray on everything but the detector, which is inside a can.
It didn't seem to have any effect. I plan to open the can and hit the
detector too. The "whiskering" as well as the method to remove it are
both kind of interesting, sort of like what happens to nicads it
seems. Well the transistors seem to have lasted forty some odd years
so I can't complain much. The NTE 160 is a germanium and it seems to
replace all of these however it does not have a fourth can lead. I
don't know if that could make much of a difference.To check one of
these for whiskering, would you simply look for leakage between any
junction and the can? I never thought of this. On another note,
considering the bias issues, I really would not want to use a silicon
transistor if I don't have to. Lenny
Michael A. Terrell
> use cold spray on everything but the detector, which is inside a can.
I may still have a couple new ECG160 in stock. I'll try to get into
the old shop and check, if I can climb through some of the junk that's
piled up to be hauled off.
http://www.nteinc.com/specs/100to199/pdf/nte160.pdf is the datasheet
for the NTE version.
I only used freeze mist as a last resort. On early transistor
equipment it seemed to cause more failures. Thermal expansion & cooling
inside the transistor case caused metal fatigue.
> It didn't seem to have any effect. I plan to open the can and hit the
> detector too.
AM or FM? A bad diode shouldn't affect both bands.
> The "whiskering" as well as the method to remove it are
> both kind of interesting, sort of like what happens to nicads it
> seems. Well the transistors seem to have lasted forty some odd years
> so I can't complain much. The NTE 160 is a germanium and it seems to
> replace all of these however it does not have a fourth can lead.
A short between the shel & the collector was a common failure.
> I don't know if that could make much of a difference. To check one of
> these for whiskering, would you simply look for leakage between any
> junction and the can? I never thought of this.
Using an analog meter would caue open juctions to start working
again, for a short time. All this did was make you waste time, since
they invariably quit again. You probably have a crack weld inside the
can, or a fractured bonding wire. That is why I monitored the collector
voltages, with added isolation and used a VTVM, and later, a FETVM.
> On another note, considering the bias issues, I really would not want
> to use a silicon transistor if I don't have to.
It's not that difficult to change. If it uses the typical two
resisotor bias, you need to raise the E_B voltage to get .6 volts
instead of .1 volts. Since the silicon parts have less leakage
currents, raise the value of the bottom resistor in the string. Tack a
trimpot or use a resistors sub box to find the new value, and solder it
in.
klem kedidelhopper
Good idea about the VTVM. I don't get to use my RCA Senior Voltohmyst
much anymore. I bought it as a kit from the RCA distributor at the
time, Bruno NY in Manhattan and built it when I was 17. It still works
great. I use an Energiser for the ohmmeter but I really do worry about
the battery leaking. I've been meaning to replace the battery with a
little isolated DC supply for a long time but just have never gotten
to it. Lenny
Ian Field
===========================================
So far I've never heard of whiskering problems with the AF12x series -
that's not to say it never happens though.
As the fault is intermittent, continuity checks may not tell you anything
useful.
Simply remove the solder from the copper pad around the screen lead and make
sure its isolated, if there is whiskering that will cure the symptom.
Ron D.
I've seen an intermittant 2AG fuse twice, Drove me nuts the first
time. By intermittant, I mean it would open in circuit, but would
test OK with an ohmmeter.
Ron D.
I mean 3AG fuse.
Phil Allison
"Ron D."
>
> I've seen an intermittant 2AG fuse twice, Drove me nuts the first
> time. By intermittant, I mean it would open in circuit, but would
> test OK with an ohmmeter.
** Yep.
Typically, the fuse wire has come loose at one end - or was never soldered
in the first place.
It looks fine and has continuity due to light pressure contact alone.
When the wire heats under load, it opens and closes.
..... Phil
Meat Plow
Start with the first RF amp. It's probably a superhet so there will be a
455 osc and mixer. After that a couple rf amps and the the detector.
Google a superhetrodyne receiver. This one uses ICs but the basics are the
same
http://www.seekic.com/uploadfile/ic-circuit/20097173226815.gif
Should give you some insight as to where the signal is being lost in the
path.
--
Live Fast Die Young, Leave A Pretty Corpse
spamtrap1888
>
> Start with the first RF amp. It's probably a superhet so there will be a
> 455 osc and mixer. After that a couple rf amps and the the detector.
>
> Google a superhetrodyne receiver. This one uses ICs but the basics are the
> same
>
> http://www.seekic.com/uploadfile/ic-circuit/20097173226815.gif
>
> Should give you some insight as to where the signal is being lost in the
> path.
How do you tune the radio in that schematic?
Phil Allison
"spamtrap1888"
>
> http://www.seekic.com/uploadfile/ic-circuit/20097173226815.gif
>
How do you tune the radio in that schematic?
** Like any receiver that uses a crystal oscillator.
By swapping the crystal.
Ask anyone with a RC model.
..... Phil
klem kedidelhopper
Well we all knew that you were off your rails Phil, but now we know
just what it was that drove you nuts. An intermittent fuse. No TWO
intermittent fuses. Possibly if not the most, then one of the most
simple components known to electronics techs. I sincerely hope they
weren't "chemical" fuses. I guess you must have slept through that
class huh? Maybe that can happen to "2AG" fuses, dunno.....Lenny
Phil Allison
"klem kedidelhopper"
** Lenny,
it would be an act of kindness to simply put a bullet in your head.
I sincerely feel sorry for anyone whose life or well being you are able to
influence - cos it must be life of pure hell for them putting up with a
lunatic like you.
Calling you as six toed, banjo plucking retard was such a masterful piece of
understatement.
Cos all you really are is a piece of fucking garbage.
.... Phil
klem kedidelhopper
So eloquently put Phil! But then I'd expect nothing less from you.
Lenny
spamtrap1888
So meatplow's proffered schematic is even more spectacularly
inappropriate to the OP's situation -- trouble shooting a nine-
transistor AM-FM radio from 1970. Which, as I recall, likely used a
variable capacitor for tuning.
Googling shows a schematic of the Transistor 305 is available for
purchase on radiomuseum.org
Ian Field
===============================
He was raised by dingoes.
Michael A. Terrell
Dingoes that were addicted to heroin.
Meat Plow
Tune as in select a frequency? Maybe by mind control? It's just a crude
example, not meant to be a service diagram.
Meat Plow
I'd call it a crude example not a fucking service manual. As I said when
I posted the image link. Here's a more relevant schematic for fuckwits
like you. I apologize but I sometimes forget there are idiots of your ilk
that need to be spoon fed and use a drool cup.
http://transistorhistory.50webs.com/lafay4.gif
klem kedidelhopper
I actually have the book with this radio. It has a schematic but comes
up somewhat short when it comes to voltages and waveforms. And the
intermittent nature of the problem makes it so hard to troubleshoot.
The other night it played on my bench for over ten hours. It never
failed and sounded fine. I finally went over and tuned the FM to a few
other stations and they seemed to sound OK. I then switched to AM and
although I did hear some stations, it being late evening I would have
expected to hear much more. But I wasn't sure about this as the shop
is in the basement. I tuned the AM between 550KHZ and 1600KHZ at which
point the radio suddenly quit. The tuning capacitor is not the issue.
And the switches don't appear to be noisy either. The scenario about
tuning the AM and the set quitting seems a bit of a stretch to be a
coincidence but then I just don't know. Anyone have any theories on if
this might be a small clue here? Thanks for following this thread,
Lenny
hrho...@att.net
wrote:
>
> - Show quoted text -
Put a scope on the output of the mixer and keep it there until the
radio dies, and see if that first stage still is working. Do this one
stage at a time until you find which stage is the problem. Have you
tried heating the radio to induce the failure, you don't mention
trying that?